In designing valve gear for internal combustion engines operating at speeds in excess of 5,000 RPM, it has been found desirable to employ valve gear of the cam-over-valve type. Valve gear of this type is known as direct-acting valve gear and employs a tappet having one end contacting the engine camshaft with the other end of the tappet in direct contact with the end of the stem of the combustion chamber valve. Direct-acting valve gear offers the advantages of low mass, fewer working parts and higher stiffness due to the elimination of the rocker arm and/or push rods. Low mass and high stiffness result in a high natural resonant frequency which allows the valve gear to attain higher RPM's before valve mismotion occurs. Direct-acting valve gear also permits the use of lighter valve spring loads for a given valve motion and engine speed as compared with those used in other valve gear arrangements. The low mass and high stiffness of the system also permits valve lift velocities and accelerations which increase the area under the valve lift curve and thus provide increased specific engine output. Although other overhead cam configurations can be made to have comparable lift velocities and accelerations, a direct-acting valve gear arrangement offers the additional advantage of permitting rotation of the cam-contacting surfaces as the lifter rotates, which is not permissible with rocker arm type valve gear arrangements. Direct-acting valve gear arrangements, therefore, allow higher permissible cam contact stresses.
In addition, the cam profile for other overhead cam valve gear arrangements with high lift accelerations and velocities is more complex than that required for direct-acting valve gear. The simpler cam profile requirement of direct-acting valve gear results in less manufacturing difficulties and less cost in the valve gear when high velocities and accelerations are desired.
Conventional lash adjusters compensate for fluid leakage by means of supplying pressurized fluid to the interior of the lash adjuster through passageways in the cylinder block. However, there are disadvantages to such an arrangement since the passageways through which the pressurized fluid flows are complicated in construction, and the operation is often unstable due to changes in the viscosity of the pressurized fluid. In order to eliminate such disadvantages, hydraulic lash adjusters of the self-contained type have been provided which are not fed from an external source of hydraulic fluid but which contain their own source of such fluid.
Self-contained lash adjusters overcome many of the shortcomings of conventional lash adjuster arrangements. Because no external source of hydraulic fluid is required, self-contained lash adjusters are easily applied to engines since no oil galleries are required. Furthermore, because no fluid is supplied to the outside diameter of the adjuster, leakage therefrom will not collect within the engine block and head as has been heretofore experienced. Because self-contained lash adjusters do not communicate with their host engine's hydraulic (lubrication) system, they do not become subject to the contaminants and air bubbles contained therein. The presence of air-free hydraulic fluid within a lash adjuster is desirable, particularly in reducing cold-start cavitation which, in the worst case, can collapse the lash adjuster. Additionally, by containing its own reservoir of hydraulic fluid, a self-contained lash adjuster has the potential for improved control over leakdown specification tolerances by selective use of hydraulic fluid having a viscosity differing from that of the host engine fluid. A still further advantage of self-contained lash adjusters is their independence of engine fluid pressure which tends to be high during cold-start conditions and low at hot idle.
Although having many advantages over conventional lash adjuster arrangements, prior art self-contained lash adjusters have a number of shortcomings. Because self-contained lash adjusters, by definition, have no outside source of hydraulic fluid, the fluid contained therein at the time of manufacture must remain intact for the life of the lash adjuster. Accordingly, a virtually perfect seal is required to prevent any self-contained lash adjuster hydraulic fluid from escaping. Providing such a seal has been the Achilles' heel of virtually all prior art commercial self-contained lash adjusters. Much of the prior art patent literature recognizes this problem and concedes that some leakage is inevitable by providing arrangements for compensating for limited amounts of hydraulic fluid loss. More specifically, the sealing problems inherent to all self-contained lash adjusters have two distinct aspects. First, all such lash adjusters require an absorption chamber to account for differential volumes of reservoir fluid. The shortcoming, in most prior art absorption chambers, lies in the attempt to establish a seal between two reciprocating elements. Such motion tends to substantially reduce the life of the seal through fatigue embrittlement and the like. The second aspect is the sealing function of the high-pressure portion of the lash adjuster. Most prior art approaches involve a dynamic or sliding seal which, by its nature, is susceptible to mechanical wear from sliding contact against less-than-perfect surface finishes. A still further problem inherent to self-contained lash adjusters is the requirement for some form of antirotation device between the lash adjuster piston and the body, which allows relative axial reciprocating motion but prevents relative rotation therebetween, to prevent torsional stressing of the interconnecting membrane seal. Furthermore, assembly of prior art self-contained lash adjusters is often complicated by the necessity to purge all air from the assembled unit. A typical manufacturing process can require assembly of the lash adjuster while submerged within hydraulic fluid.
It has been found difficult to provide direct-acting valve gear in engine applications where the height of the engine must be kept to a minimum and, consequently, the camshaft located closely adjacent the end of the combustion chamber valve stem. Furthermore, where it is desired to retrofit a hydraulic lash-adjusting tappet into the direct-acting valve gear of a production engine, it is often difficult to provide a hydraulic lash-adjusting tappet in the space provided between the camshaft and the end of the valve stem. Since the tappet must be guided in the bore defined by engine structure intermediate the camshaft and the end of the valve stem the engine height tends to somewhat increase.
Therefore, it has been desired to find a self-contained hydraulic lash-adjusting tappet with a compact profile height for use in engines having direct-acting valve gear with minimum distance between the camshaft and the end of the valve stem to minimize the mass of engine structure necessary to provide the tappet guides. Furthermore, in designing tappets for direct-acting valve gear so as to minimize sideloading in the guide for minimizing wear, it is desirable to have the reaction force of the valve stem centered through the tappet at a point as closely adjacent the cam surface as possible. Locating the reaction force near the cam face also permits the tappet to be designed to minimize the mass which, in turn, reduces inertia.
Known hydraulic tappets for self-contained direct-acting valve gear have employed a body or bucket, formed as an integral unit having a reservoir defined by the closed end of the body and an annular diaphragm, such as that shown and described in U.S. Pat. No. 3,521,608 to Scheibe, wherein the diaphragm is retained about the outer circumference thereof to the body and engages the plunger portion of the lash adjuster at the inner circumference thereof. Although providing a bucket type self-contained lash adjuster with a relatively small profile, seal arrangements such as that shown in the Scheibe lifter can have shortcomings when the device is applied to certain applications, particularly those requiring long life and minimal hydraulic fluid leakage. Such a device overcomes some of the above-described shortcomings of other prior art devices by eliminating need for a dynamic seal. However, the requirement of a fluid-tight absorption chamber requires life-long seal integrity. In the applicants' experience, problems in prior art designs of this type often arise in the area of interface between the valve stem and plunger assembly. Because the lash adjusting mechanism axially reciprocates at this point, seals tend to deteriorate rapidly by pulling away from a host member or embrittle and rupture at a point of maximum excursion.